The Subject terms are designed to better group legislation, to improve consistency, and to shorten the time it takes to add Subject terms to bills. 
Beginning with the 111th Congress, all legislative measures are assigned at least one Subject term drawn from a new list of terms. The Subject terms are designed to better group legislation, to improve consistency, and to shorten the time it takes to add Subject terms to bills. Subject terms can also be assigned from a large list of terms corresponding to geographic areas (foreign and domestic), U.S. government entities, and congressional committees. See the CRS Named-Entity Subject Terms for a complete list of those terms. Subject term assignment is cumulative; that is, it is added to in order to reflect revisions (if any) made to the measure as it moves through the legislative process.

When looking at the Subject terms assigned to a bill or resolution, you will notice that the first term on the list appears out of alphabetical order. This is the Top Term for the measure, and it corresponds to the broad category that best describes the concerns of the measure's text. Top Terms are represented by the bold "umbrella" terms on the list.